Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/38—Improvement of the adhesion between the insulating substrate and the metal
  • H05K3/386—Improvement of the adhesion between the insulating substrate and the metal by the use of an organic polymeric bonding layer, e.g. adhesive
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/46—Manufacturing multilayer circuits
  • H05K3/4611—Manufacturing multilayer circuits by laminating two or more circuit boards
  • H05K3/4626—Manufacturing multilayer circuits by laminating two or more circuit boards characterised by the insulating layers or materials
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K2201/00—Indexing scheme relating to printed circuits covered by H05K1/00
  • H05K2201/01—Dielectrics
  • H05K2201/0183—Dielectric layers
  • H05K2201/0195—Dielectric or adhesive layers comprising a plurality of layers, e.g. in a multilayer structure
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
  • H05K2203/05—Patterning and lithography; Masks; Details of resist
  • H05K2203/0562—Details of resist
  • H05K2203/0571—Dual purpose resist, e.g. etch resist used as solder resist, solder resist used as plating resist
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/0011—Working of insulating substrates or insulating layers
  • H05K3/0017—Etching of the substrate by chemical or physical means
  • H05K3/0023—Etching of the substrate by chemical or physical means by exposure and development of a photosensitive insulating layer
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/02—Apparatus or processes for manufacturing printed circuits in which the conductive material is applied to the surface of the insulating support and is thereafter removed from such areas of the surface which are not intended for current conducting or shielding
  • H05K3/06—Apparatus or processes for manufacturing printed circuits in which the conductive material is applied to the surface of the insulating support and is thereafter removed from such areas of the surface which are not intended for current conducting or shielding the conductive material being removed chemically or electrolytically, e.g. by photo-etch process
  • H05K3/061—Etching masks
  • H05K3/064—Photoresists

Definitions

• This invention relates to the manufacture of multilayer printed circuit boards (PCBs) .
• the bare board is often a multi ⁇ layer board, generally comprising a first, inner-layer which is usually an epoxy-bonded fibreglass sub-layer clad on one or more usually both sides, with a conducting sub ⁇ layer.
• the conducting sub-layer usually comprises conducting material which is metal foil and most usually copper.
• the conducting material is generally a discontinuous sub-layer in the form of the circuit pattern.
• an outer (or further) layer is attached on at least one, and generally on both sides of the inner layer.
• An outer layer will comprise at least a layer of an insulating material, again, generally epoxy-bonded fibreglass which is bonded to the conducting sub-layer of the innerlayer.
• the outer layer may additionally comprise at least one conducting sub-layer.
• the outer layer or layers are insulating layers or are adhered to the conducting sub ⁇ layer of the first inner layer with an insulating layer or sub-layer innermost to the first layer.
• a multi-layer board therefore comprises a laminate in which conducting sub-layers are separated by insulating layers or sub-layers.
• the laminates are formed by adhering layers together.
• the layers which form the laminate may be conducting or insulating only or more usually are made up from sub-layers: at least one insulating and at least one conducting sub-layer.
• conducting sub-layers are only provided on an insulating sub-layer due to their fine, fragile nature.
• the inner-layer (or first layer) comprises an insulating sub-layer having a conducting sub-layer on each side and the outer layers (or second layers) are insulating only.
• the conducting sub-layers in this example comprise copper foil.
• the insulating layers in this example are epoxy- bonded fibreglass.
• the material of the outer layers is known as pre-preg and generally comprises a higher proportion of epoxy resin relative to the fibre glass than the sub-layer of the inner layer.
• the inner-layer 1, (illustrated in Figure 1A) , comprising insulating sub-layer 2 and copper foil layers, 3 is coated on each side with a light-sensitive film 4 (shown in Figure IB) which is exposed to light in pre ⁇ selected areas.
• the unexposed film is then removed by developing to leave a pre-selected pattern of exposed copper 5, as illustrated in Figure 1C.
• the areas of the copper which are revealed are generally the areas which are to be etched away, to leave the desired circuit pattern of the copper in the pattern corresponding to the cured photo ⁇ resist.
• the unwanted copper areas are then etched away using a chemical etchant, resulting in the arrangement illustrated in Figure ID.
• the photo-resist remaining is chemically removed to result in the arrangement illustrated in Figure IE and the revealed copper surface is cleaned to remove any tarnishing from the copper surface.
• a black or brown oxide layer 6 is then formed on the cleaned copper surface (as shown in Figure IF) and then as shown in Figure 1G the two outer layers 8 are then placed onto the black or brown oxide coatings on each side of the inner-layer, respectively.
• the laminate is then formed by pressing the layers together and applying heat so that the insulating sub-layer (pre-preg material) of outer layers 8 flows and provides adhesion to the brown or black oxide coating.
• Additional conducting sub-layers may also be added to the laminate.
• an additional layer comprising a conducting sub-layer, generally copper which has been pre-treated by a passivation step for example a zinc, tin, chromium or molybdenum passivation step is placed onto the outside of the pre-preg, prior to applying prressure and heat.
• a passivation step for example a zinc, tin, chromium or molybdenum passivation step
• the pre-preg adheres to the inner- layer and the additional layers adhere to the pre-preg substantially simultaneously.
• the exposed copper surface which is in the form of a circuit pattern is cleaned to remove the tarnish layer and then the copper surface is immersed in hot, strongly alkaline, hypochlorite solution generally at temperatures of around 40 to 110°C.
• this process may be problematic because the formation of the black oxide layer must be carefully controlled to ensure formation of a coating which will give good adhesion after lamination.
• these black oxide layers are subject to attack by solutions which dissolve copper oxide such as the pre-treatment solutions for the electroless plating processes which are often acidic.
• the dissolution of the copper oxide may result in delamination. This problem is known as "pink ring" because a pink ring of copper can be seen in the black oxide coating on the copper.
• meth(aerylate)-based resins for forming the photo-resist, for example, see GB-A-2193730.
• the photo-resist compositions described in this reference are for use in conventional processes where the resist is removed prior to cleaning of the copper surface and preparation for lamination as described above.
• epoxy-acrylate- or aerylate- based resins for forming a solder mask. The solder mask is used at a later stage in the bare board manufacture when the bare board is almost complete and the outer surfaces are being prepared for passing to the next component attachment stage.
• solder mask are described in "Fine Line Resolution Solder Mask" by S. Rodriguez and K. Cheethan published in the Proceedings of the EIPC Winter Conference (PCB Applications: New Processes, OEM and Far East Challenges) Zurich 11.12.91 p. 3-4-1 to 3-4-7.
• EP-A-51,630 also describes an epoxy-acrylate solder mask composition and EP-A-515,861 describes epoxy-based inks which are used to form a protective coating on circuit boards.
• the resist or mask provides a protective function only and in particular, the photo-resist after having fulfilled its protective function is removed.
• photo ⁇ resists have been developed which after etching of the copper, does not have to be removed, but can remain in place.
• the pre-preg is then adhered to the remaining photo-resist in the adhesion step.
• Such photo-resists are known as permanent or bonding resists.
• the bonding resists disclosed in each of these references have significant limitations. They have been found to provide PCBs which tend to delaminate. It is thought that this is largely due to "outgassing" on use whereby gases, generally water vapour, become trapped between the layers of the laminate either at the boundary between resist and metal layers, or within the resist itself in particular in humid conditions. On subsequent heat treatment of laminated PCBs for example in a soldering step, outgassing occurs resulting in delamination of the board and/or damage to plated through-holes in the board.
• the known permanent (or bonding) resists require organic solvent developing to remove the non-fixed areas of resin in the developing step after UV curing of the resin.
• organic solvent developing is disadvantageous both because of the safety problems involved and the environmental costs concerned.
• the present invention aims to overcome the problems of the prior art systems for multi-layer board manufacture.
• the present invention provides a process for manufacturing a multi-layer circuit board comprising obtaining a first layer which comprises at least one insulating sub-layer and at least one conducting sub-layer; applying a layer of a resin composition to the surface of the conducting sub-layer; in a fixing step, fixing the resin composition in a pre-selected pattern to form selectively a fixed pattern of resin composition which is etch-resistant and a non-fixed pattern of resin composition; then in a developing step, contacting the non-fixed pattern of resin composition with an aqueous developing solution to effect removal of the non-fixed pattern of resin thereby revealing the areas of conducting sub-layer for etching; etching the revealed areas of conducting sub-layer by contact with an etchant; contacting an insulating second layer with the fixed, etch-resistant resin composition; and in a final adhesion step, adhering the first and second layers to one another.
• a multi-layer circuit board comprising a first layer, etch-resistant resin and an insulating second layer in which the first layer comprises an insulating sub ⁇ layer and a conducting sub-layer, the first layer being adhered to an insulating second layer so that the etch- resistant resin composition is in direct contact with both the conducting sub-layer and the insulating second layer, and wherein the etch-resistant resin composition is formed from a resin composition comprising a first resin component comprising photopolymerisable groups and having an acid number of from 15 to 75, a second resin component comprising unreacted epoxy groups and a photoinitiator, the weight ratio of first resin component to second resin component being at least 1:1, preferably at least 3:2.
• a multi-layer board having good adhesion can be formed in a considerably simplified process.
• the multilayer boards produced have good adhesion even on exposure to thermal shock conditions, such as in one or more subsequent soldering steps.
• the process of the present invention enables the etch-resistant resin, generally a photo-resist, to remain in place and to take part in the adhesion of the two layers so that the process steps involved in forming a multi-layer PCB can be considerably reduced: steps of removing the etch-resistant resin, cleaning the exposed copper surfaces, and applying a primer (either copper oxide or some other layer) prior to laminating the layers together, can all be omitted.
• Organic solvents are not required in the developing step, thereby reducing safety and waste disposal problems.
• the process according to the present invention does not require the use of a copper oxide layer, the problem of "pink ring" in the prior art processes does not arise as the adhesion promoter is resistant to the plating processes.
• the first layer is the inner-layer of a circuit board and may comprise one insulating sub-layer having a conducting sub-layer (generally copper foil) on one side only, or on each side.
• the insulating second layer is sometimes referred to as an outer layer in the PCB industry, generally being a prepreg layer.
• the bonding resist may be applied to both conducting sub-layers as described in the process above and two outer layers may be applied, one to each of the conducting sub-layers.
• the general principle is that conducting layers and insulating layers should be alternate.
• the conducting sub-layers in the finished multi-layer board will generally not comprise continuous conducting material as they are generally the discontinuous pattern of conducting material remaining after etching, in the form of a circuit pattern.
• the resin composition is applied to the surface of the conducting sub-layer by any conventional means.
• the resin composition will be in a liquid form, for example having a viscosity of from 50 to 10,000, preferably greater than 100 and most preferably greater than 500 cps.
• the viscosity will generally be below 2000, or even 1500 cps.
• Liquid resin compositions can be applied by any conventional coating technique such as screen printing, roller coating, electrostatic spraying or curtain coating. The appropriate viscosity can be selected for the chosen application method, or vice versa. More viscous liquids are used for screen printing than for example spaying, dip coating or curtain coating application techniques.
• the composition may be applied in a dry form, for example a pre-formed layer may be prepared on a support film and then adhered to the conducting layer in a method conventional for photo-resists, for example as described in GB 2,193,730, the supporting layer then being removed.
• the conducting sub-layer undergoes a pre-treatment cleaning step.
• the cleaning step may be either a mechanical cleaning step such as brushing or pumice scrubbing, or a chemical step in which the conducting sub-layer is contacted with a chemical cleaning composition which is generally an acidic composition.
• the dry coating thickness of the resin composition is generally from 3 ⁇ m to 50 ⁇ m, preferably from 7 ⁇ m to 20. It has been found that at dry resin thicknesses below 7 ⁇ m, the cover over the conducting sub-layer may be unreliable so that the conducting layer may break through. Whilst resist thickness of greater than 20 ⁇ m may be provided, they have not been shown to give any additional benefit and may tend to cause bond failure after formation of multi-layer laminate. Preferably, the coating thickness will be at least 9 ⁇ m and most preferably at least lO ⁇ m.
• the thickness of the resin composition will be no greater than 18 ⁇ m, preferably no greater than 15 ⁇ m.
• the drying step is generally carried out by exposing the coated first layer to hot air, for example by passing coated first layers through a forced air dryer, or otherwise exposing to forced air drying.
• the drying temperature will be no greater than 120 ⁇ C, preferably no greater than 100°C and most preferably mo greater than 90 ⁇ C and should not be so high and/or for such length of time to activate the adhesion step prematurely.
• the resin composition is fixed to form selectively non-fixed and fixed etch-resistant areas by selective exposure to or contact with a reaction promoter for the resin composition.
• the resin composition is a radiation- curable composition and the reaction promoter is radiation, such as electron beam curing or actinic radiation and most preferably UV light.
• the fixed, etch-resistant pattern preferably forms in the areas which are contacted with or exposed to the reaction promoter so that the areas not contacted or exposed to the reaction promoter form the non- fixed pattern. This is a conventional negative working process.
• the fixed etch-resistant pattern may form from the areas of the resin composition which are not contacted with or exposed to the reaction promoter.
• the reaction promoter produces a reaction in the areas of resin composition which are exposed or contacted with it, the reaction resulting in those areas of the resin composition becoming soluble in a developing liquid used in the developing step for removing the non-fixed pattern. This latter process is known as positive working.
• the fixing step is a negative working step where the reaction promoter produces polymerisation or any other curing which fixes the areas of the resin composition exposed to, or contacted with the reaction promoter.
• the developing step the non-fixed areas of the resin composition are removed leaving a pattern of fixed, etch-resistant resin composition and a pattern of exposed conducting sub-layer.
• Alkaline developing and solvent developing are well known developing techniques for conventional photo-resists which are stripped off prior to adhesion of various layers to form a multilayer or PCB.
• Alkaline developing solutions are aqueous and may comprise a weak alkali such as for example potassium carbonate or a solvent developing solutions may comprise a solvent such as butyl diglycol.
• a solvent developing is given in EP-A-514,630.
• An example of aqueous alkaline development is described by Rodriguez and Cheetham
• the developing solutions may be aqueous alkaline developing solutions, comprising one or more weakly alkaline salts such as alkali metal carbonates, preferably potassium or sodium carbonate.
• the concentration of the weakly alkaline salt will be from 0.5-20% by weight, generally around 0.5-5% by weight in the aqueous developing solution.
• the developing solution generally contacts at least the non-fixed areas of the dry resin composition. Contact may be by any conventional means but is generally by immersion or spraying with the developing solution, generally by spraying.
• the developing step is carried out for sufficient time to enable the non- fixed areas of the resin composition to be removed from the conducting sub-layer, revealing the areas of conducting sub-layer for etching and the fixed (cured) pattern of resin composition which is etch-resistant.
• a multi-layer circuit board is manufactured, the process comprising: obtaining a first layer which comprises at least one insulating sub-layer and at least one conducting sub-layer; applying a layer of a resin composition to the surface of the conducting sub-layer; in a fixing step, exposing the resin composition in a pre ⁇ selected pattern to light, preferably UV light, to produce a reaction in the resin composition which forms a fixed pattern of resin composition in the areas exposed to light and a non-fixed pattern of resin composition remaining in the areas of the resin composition which have not been exposed to light; then in a developing step, contacting the non-fixed pattern of resin composition with an aqueous developing solution to effect removal of the non-fixed pattern of resin thereby revealing the areas of conducting sub-layer for etching; etching the revealed areas of conducting sub-layer by contact with an etchant
• a post-development bake step may be included in the process to increase hardness of the fixed areas of photo-resist resin. This may be necessary to increase the etch-resistance of the resin prior to contact of the assembly with etchant to remove the unwanted areas of conducting sub-layer.
• the post-development bake does not reach temperatures which will substantially activate any latent catalyst which may be present in the resin composition for the adhesion step, or otherwise activate the resin composition prematurely so that it will not adhere in the subsequent adhesion step.
• the post-development bake should not exceed temperatures above 100 ⁇ C, most preferably it should not exceed temperatures above 80 ⁇ C.
• the fixed areas of resin composition Prior to contact with the etchant, the fixed areas of resin composition will be etch-resistant so that they are resistant to an etchant at least for the time taken to etch away the unwanted areas of conducting sub-layers to form the circuit pattern of the first layer.
• etchant Several different chemical types of etchant are known and used; preferably the etch-resistant resin composition is resistant to more than one type of etchant or even to all types.
• a preferred etch-resistant composition is resistant to acidic etchant which may comprise for example iron II chloride or copper chloride.
• An additional preferred composition is resistant to alkaline etchant which may be for example based on ammonium compounds, such as a copper ammonium chloride complex in aqueous ammonia.
• the second layer(s) generally one or more prepreg layers are placed adjacent the etch-resistant composition which remains in place on top of the remaining pattern of conducting sub-layer and the two layers adhered to one another in the adhesion step.
• the composite is exposed to heat in an adhesion step which initiates the adhesion reaction.
• the bond strength between the copper or other conducting sub-layer is generally enhanced due to enhanced cross-linking within the resin; a bond forms between the resin composition and insulating, second layer both due to reaction in the resin composition, generally cross-linking, and mechanical bonding due to intermixing of the resin composition and the insulating, second layer due to any heat applied in the adhesion step.
• pressure is also applied by placing layers which are to be formed into a multi-layer laminate of at least the first and second layers, in a press. Where pressure is applied it is generally from 100 to 400 psi, preferably from 150 to 300 psi.
• the temperature of this adhesion step will generally be from 100 ⁇ C to 250 ⁇ C, preferably from 120 ⁇ C to 200°C.
• the adhesion step is generally carried out for any time from 5 minutes to 3 hours, most usually from 20 minutes to 1 hour, but is for sufficient time and pressure and at a sufficiently high temperature to ensure good adhesion between the first and second layers.
• the resin of the insulating sub-layers (generally epoxy resin) tends to flow ensuring that the areas between the conducting areas of the conducting metal sub-layer revealing the insulating sub ⁇ layer of the first layer are filled. This ensures that the conducting sub-layer material is substantially sealed between insulating layers and subsequent penetration of water or air is avoided.
• any latent catalyst is activated so that further hardening of the resin, for example by cross-linking or polymerisation reaction occurs in the resin composition. This ensures good adhesion between the conducting material of the conducting sub- layers and the insulating layer adjacent to it, forming a strong bond between the adjacent layers.
• the present invention provides a considerably simplified process over the known processes and provides good protection of the copper or other conducting material and good adhesion, whilst overcoming the prior art problems of pink ring.
• the present invention also includes a resin composition for forming a bonding resist and which is preferably used in the process of the present invention, the resin composition comprising a first resin component comprising photopolymerisable groups and having an acid number of from 15 to 75, a second resin component comprising unreacted epoxy groups, and a photoinitiator, the weight ratio of the first resin component to the second resin component being at least 1:1, preferably at least 3:2.
• the composition optionally also comprises a latent catalyst.
• the acid number of the first resin component is at least 25, more preferably at least 30.
• the acid number is no greater than 65 and most preferably no greater than 60.
• R groups are selected independently from H, C ⁇ alkyl groups and halogens, although halogens are not preferred.
• Preferably all three R groups are hydrogen for example in aerylate end groups, or one R group is CH 3 , for example as in methacrylate end groups.
• the alkyl groups may be optionally substituted, for example with halogens such as chlorine or bromine.
• the first resin component which comprises photo ⁇ polymerisable groups is preferably an epoxyacrylate or epoxy (C,-. ⁇ )aerylate (preferably epoxymethacrylate) or an epoxy resin which is an aromatic polyglycidyl ether having the structural
• Ar is an optionally substituted aromatic group having from 1 to 4 phenyl rings
• R is a divalent hydrocarbon group, preferably having from 1 to 10 carbon atoms, most preferably having from 1 to
• R 1 is a combination of H, acidic functional groups and ethylenically unsaturated groups, wherein R is H for no greater than 30% of the total number of R groups, R 1 is an acidic functional group in sufficient proportion to provide an acid number of from 15 to 75, most preferably from 25 to 65, or even from 30 to 60, and the remainder of the R groups being ethylenically unsaturated groups; and x is preferably from 2 to 10.
• the first resin component comprising photo- polymerisable groups has the formula I above, preferably up to 30% of the R groups are H; from 4-95% of the R groups are acidic functional groups and from 4-95% of the R groups are ethylenically unsaturated groups.
• R groups are acidic functional groups and ethylenically unsaturated groups, respectively.
• the first resin component is substantially free of unreacted epoxide groups.
• the first resin component is preferably an epoxy resin
• the first resin component is preferably the reaction product of an epoxy compound so that substantially all the available epoxy groups were reacted during its formation.
• the molecular weight of the photo-polymerisable resin is preferably no greater than 10,000 and most preferably at least 500 and no greater than 5000, or even no greater than 3000 (as determined by GPC) .
• Suitable (meth)aerylate resins are any conventionally used for forming conventional photo-resists or solder masks.
• Suitable examples include unsaturated esters of an epoxy novolac resins are used where the unsaturated acid is (meth)acrylic acid, halogenated (meth)acrylic acid or hydroxyalkyl(meth)acrylate half esters of a di-acid.
• the photo-initiator is incorporated in the composition to promote reaction of the ethylenically unsaturated groups in the photo-initiated step.
• the preferred photo- initiators are those which on irradiation, give an excited state that leads to formation of free radicals which then initiate polymerisation of the monomer, such as organic peroxides, hydrogen peroxides, alpha-substituted acetophenones such as 2,2-trichloro-4 -tertiarybutyl acetphenone, benzoylphenylcarbonols and alkyl ethers thereof such as benzone and its alkyl ethers, benzophenones, benzil, acetals and mixtures of phenothiazine dyes or quinoxalines with electron donors.
• Preferred photoinitiators are benzophenones and/or acyl phosphines.
• the photoinitiator is generally incorporated in the composition in amounts of from 0.5 to 15% by weight more usually from 1 to 10% by weight of the total resin composition.
• the second resin component comprises an epoxy resin having unreacted epoxide groups which, in the adhesion step, on conditions of high temperature react to increase adhesion to the surface of the conducting sub-layer (generally copper) and to give good adhesion to the insulating, second layer.
• the epoxy resin is any resin which contains unreacted epoxide groups and may be for example one or mixtures of more than one of epoxy-novolac resins, bisphenyl A glycidyl ether prepolymers, bisphenol F diglycidyl ether prepolymers or bisphenol H diglycidyl either prepolymers or epoxy- novolac resins as descried in EP-A-514630.
• the softening point of the second resin component is preferably from 60-90°C.
• the resin composition for use in the present invention should be chemically compatible with the insulating layers in the final laminate produced by adhering the layers together.
• Epoxy groups are the preferred functional groups for reacting in the adhesion step, and for forming the backbone in the first resin component because the resin contained in the second insulating layer (prepreg) and often also or sub-layers is generally epoxy-based and therefore the use of epoxy functionality ensures chemical compatibility during the bonding process. Furthermore, epoxy resins are known to have good thermal stability and electrical insulation properties.
• the first and second resin components are generally incorporated in the composition of the invention in weight ratios of 2:1 to 6:1, preferably 2:1 to 4:1 and most preferably around 3:1.
• the composition of the invention may optionally also comprise one, or mixtures of more than one additional photopolymerisable components to increase the cross-linking density in the resin composition during the adhesion step.
• the additional photopolymerisable component may be incorporated in the resin composition in amounts up to 25% by weight, but preferably in an amount no greater than 20% or most preferably in an amount below 15% by weight of the total composition.
• Any photopolymerisable monomer, di er or oligomer may be used but particularly preferred are multi-functional compounds, such as (meth) acrylates such as TMPEOTA (tri ethylol propane ethoxylate triacrylate) and/or TPGDA (tripropylene glycol triacrylate) and/or RCS 88'999 (a urethane acrylate resin) and/or Santolink AM 1150 (melamine acrylate) .
• TMPEOTA tri ethylol propane ethoxylate triacrylate
• TPGDA tripropylene glycol triacrylate
• RCS 88'999 a urethane acrylate resin
• Santolink AM 1150 melamine acrylate
• composition will generally contain components to provide a ratio of photopolymerisable to adhesion reactive epoxy groups of from 2:1 to 10:1, preferably 2:1 to 5:1 and most preferably around 3:1.
• the composition preferably comprises 50 - 94% by weight first resin component and 5 to 49% by weight second resin component. More preferably, the composition should comprise from 50 to 89% by weight first resin component and from 10 to 49% second resin component.
• the etch-resistant resin composition therefore preferably comprises an organic resin which has reacted in a polymerisation step and a cross-linking step, and in which at least some of the polymerisation or cross-linking has been initiated by exposure to radiation.
• Particularly preferred etch-resistant resin compositions are those where the cross-linking of the resin is due to reaction of acid groups of an epoxy(meth) acrylate resin with the epoxide groups on a second resin component.
• ethylenically unsaturated groups react to increase the average molecular weight of the resin composition producing the fixed areas.
• the acidic groups in the resin composition provide aqueous developability and in the developing step, the lower molecular weight non-fixed areas are therefore developed away leaving the fixed areas in place on the conducting sub-layer.
• reaction between the acid groups of the first resin component and the epoxy groups of the second resin component takes place to form a strong bond between the layers.
• the acidic groups in the resin composition have been found to ensure good contact to the conducting metal sub-layer.
• the laminates produced are resistant to delamination even under heat shock conditions and compared with the prior art systems do not suffer from the problem of outgassing at problematic levels. It is postulated that the particularly good,properties are due to the fact that reaction occurs between the two specified resin components which are in an intimate mixture, therefore ensuring reaction throughout the composition and reducing weak spots in the resist and ensuring that substantially no unreacted epoxide groups remain in the bonding resist. This, combined with the acidic nature of the resin composition ensures a particularly effective contact between the surface of the conducting sub-layer and the composition, which appears to minimise moisture uptake, thereby reducing the problem of out-gassing.
• out-gassing in the prior art bonding resists may be due at least in part to unreacted epoxide groups in the resin composition which have an affinity for water.
• the preferred compositions comprise a first resin component having functional groups effective to photochemically react on exposure to a reaction promoter which is preferably UV radiation, in the fixing step and acidic groups effective to provide aqueous developability in the developing step; photoinitiator for the photo- initiated reaction; a second resin component comprising unreacted epoxide groups effective to react with acid groups on the first resin component in the adhesion step, and optionally latent catalyst to increase the rate of reaction in the adhesion step.
• the composition preferably also comprises a latent catalyst for the adhesion step, to increase the rate of reaction in the adhesion step.
• the optional latent catalyst is preferably incorporated in amounts up to 5% by weight, most preferably from 0.5 to 4% by weight.
• Suitable latent catalysts are catalysts for the reaction between the epoxy groups of the second resin component and the acid groups of the first resin component. Examples are blocked catalysts such as amine salts of sulphonic acids eg. morpholene salt of p- sulphonic acid.
• a "latent" catalyst is one which remains substantially inactive during the first, photo-initiated polymerisation step, and does not become active as a catalyst for the polymerisation of the functional groups for reaction in the adhesion step, until the adhesion step.
• activation of the latent catalyst in the second polymerisation step is by heating at temperatures of above 120°C, or even above 140°C.
• the resin composition which may be used in the present invention is generally prepared by mixing the resin components, photoinitiator and any other optional ingredients to form a substantially uniform composition, generally with solvent, such as glycol ethers or glycol ether acetates. Where necessary, fillers and other components may be milled either prior to incorporation in the mixture, which is preferred, or after addition, by milling the final composition.
• the resin composition may also include binder, pigment defoaming agents and other non-interfering additives or fillers.
• composition was prepared by forming a mixture of the following ingredients in the parts by weight quoted in Table 1:
• the components were stirred to form a substantially uniform composition.
• the composition was then coated onto a pre-cleaned copper foil sheet of an inner-layer for a circuit board by screen printing and the coating was dried to drive off solvent in circulating air at a temperature of 80 ⁇ C for approx 10 minutes to leave a dried coating, having a thickness of 14 ⁇ m.
• Pre-selected areas of the composition were then exposed to UV light by covering with a negative and passing through a UV-oven so that a pre-selected pattern of the coating only was exposed to UV-light of an intensity of 5 KW for 30 seconds.
• the unexposed areas of the film were then removed by alkali-developing by immersing in a 2% aqueous solution of sodium carbonate at a temperature of 30 ⁇ C for 30 seconds.
• the exposed copper areas were then etched using a standard etching composition of ferric chloride and the etched inner layer was washed with water and dried.
• the inner layer was then dried by forced air drying, and the neighbouring layer of the multi-layer board was placed on the remaining photo-resist, with the insulating layer inner most.
• the two adjacent layers were then placed in a press and a pressure of 250 psi was applied and heat was applied at 150 ⁇ C for 1 hour.
• the resulting composite had good adhesion in which the resin in the insulating layer had flowed to form a seal between the two layers.
• a bonding resist composition was prepared by forming the mixture of components as set out in Table l of example 1 with the exception that the 30 parts by weight TMPEOTA was replaced with 30 parts by weight TPGDA UV-curable monomer.
• the composition shows a slower UV-response than the composition of example 1 because the TPGDA is difunctional rather than trifunctional (TMPEDTA) .
• TPGDA is difunctional rather than trifunctional
• curing is still achieved by exposure to UV-light of an intensity of 5KW for 40 seconds. Good bonding resist properties were obtained.
• a bonding resist composition was prepared as described in example 1, with the exception that the Novapint Violett A511 pigment was replaced with 2.0 parts by weight. Savinyl Violett*" pigment (supplied by Sandoz) , and the amount of Syloid ED20 ** of filler was increased from 15 to 30 pbw. Again good bonding resist properties were obtained when the composition was used to form a multi-layer board, as described in example 1. It was noticed that dye was adsorbed onto the copper areas during developing of the resist and because of the increased amount of filler, the surface of the resist had a more matt appearance than that of the resists formed as described in examples 1 and 2, respectively.
• a bonding resist composition was prepared as described in example 1 but with the exception that Albiset B202 was replaced with Albiset XP 9/14. Again, the composition formed an effective bonding resist. It was noted that the composition formed produced a somewhat harder film on drying and required a developing time slightly longer (by 10-15%) than the developing time required for the composition of example 1.
• a composition was prepared as defined in example 1 but with the exception that the 480 pbw epoxyacrylate resin (Albiset A210 (65% active)) of example 1 were replaced with 450 pbw of an epoxymethacrylate resin (PRO 1200 (70% active) supplied by Cray Valley. It was noted that the composition gave a slightly slower UV-response than the composition of example 1. However, cure was still obtained on exposure to UV-light of an intensity of 5 KW for 30 seconds. Good bonding resist properties were obtained.
• Example 6 Performance tests were carried out on a composition as described in table 1 of example 1, as follows: Pretreatment 3 sets of innerlayer panels were given different surface preparation as follows:
• Exposure was done using a vacuum frame equipment with a 3KW source.
• Optimum exposure time was determined using a Dynachem step wedge, and was found to be about 30 light units (equivalent to 1 min.). This gave a break on the step wedge after development of about 5/6. It was noted that there was only a small change in the break point on the step wedge with a wide range of exposure times.
• the panels were spray developed in 1% carbonate at 30°C. Etching
• the multi-layer press was a standard Burkle type without vacuum.
• the press temperature was 171°C with a ram pressure of 71 bar (equivalent to 250 psi on the panels) .
• the overall press cycle was about 1% hours with 1 hour at maximum temperature and pressure. After bonding the outer copper was etched away on two panels for the following thermal shock test. The remainder of the panels were sent for drilling and plating. Thermal Shock One baked and one unbaked panel were tested by repeating cycling in a vertical hot air leveller (solder pot temperature 240 ⁇ C) . After 30 seconds of levelling neither panel showed any sign of delamination. The baked panel was cycled for a further 30 seconds and still showed no signs of delamination.
• composition A as described in example 4, but with the Albiset A210 replaced by the equivalent weight of Albiset
• the 0.7mm insulating layer inner layers were pretreated by the mechanical brushing; and the 0.36mm and 0.2mm insulating layer inner layers were all pumice scrubbed.
• the panels were printed using the respective compositions A and B, by screen printing using a 100 T polyester mesh. In each case, the screen was double flooded before printing with either composition A or composition B. The panels were dried at 65-70°C (circulating air oven) , for 10 minutes on the first side and eight minutes on the second side.
• 65-70°C circulating air oven
• the panels were developed using a 1% sodium carbonate solution at 32°C. The panels were then developed at conveyor speed 320mm/minute. Etching
• the treated inner layers were then bonded in a Burkle open press with the following cycle: four minute gel time at 170 ⁇ C and 10 bar ram pressure; 70 minute bond time at 170°C and 71 bar ram pressure; and 30 minutes cool down time.
• the final mutlilayer boards were constructed as shown below.
• the four layer multilayer according to the invention was prepared using inner layers having photoresist formed from compositions A and B respectively.
• the four layer board was then given repeated hot air solder levelling in a Ce co Quick Silver machine at 252°C. After 37 seconds of solder immersion, neither board showed signs of delamination. From a comparison of the results obtained using the comparative four layer multilayer, it was concluded that the copper-bonding resist-prepreg bond gives very good adhesion which may be as strong as the copper- prepreg bond in the comparative example.
• the bond obtained using composition A was particularly good, although no further testing was carried out to determine whether the bond obtained using composition A was stronger than that between copper and the pre-preg in the comparative example.

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• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Production Of Multi-Layered Print Wiring Board (AREA)
• Manufacturing Of Printed Circuit Boards (AREA)
• Superconductors And Manufacturing Methods Therefor (AREA)
• Reinforcement Elements For Buildings (AREA)
• Laminated Bodies (AREA)
• Materials For Photolithography (AREA)
• Non-Metallic Protective Coatings For Printed Circuits (AREA)

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• 1994
  • 1994-10-13 TW TW083109523A patent/TW290583B/zh active
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  • 1994-10-14 WO PCT/GB1994/002258 patent/WO1995010931A1/en active IP Right Grant
• 1997
  • 1997-11-24 US US08/976,906 patent/US6074803A/en not_active Expired - Fee Related
• 1998
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